Van Leeuwenoek: First microbiologist





“For me this was among all the marvels that I have discovered in nature the most marvelous of all, and I must say that, for my part, no more pleasant sight has yet met my eye than this of so many thousands of living creatures in one small drop of water, all huddling and moving, but each creature having its own motion.”

Protists: The big picture


  • All Eukaryotes are protists except plants, animals and fungi
    • mostly unicellular
    • amoeba, slime molds, algae, protozoa


  • Extraordinarily diverse
    • can be smaller than prokaryotes
    • functions carried out by organelles
    • membrane bound nucleus


  • First undisputed fossils ~1.8 bya
    • spherical algal protists
    • many protists no hard parts
    • ancestry unresolved

Basic protist cell: Euglena

How did Eukaryotic cells evolve?

  • Origin of the nuclear membrane?


  • Origin of mitochondria?
    • All eukaryotic cells have mitochondria
    • so mitochondria is common ancestor?


  • Origin of chloroplasts?
    • Photosynthetic eukaryotes have chloroplasts
    • some from common ancestor
    • others….?


How did Eukaryotic cells evolve?

  • Origin of the nuclear membrane?


  • Origin of mitochondria?
    • All eukaryotic cells have mitochondria
    • so mitochondria is common ancestor?


  • Origin of chloroplasts?
    • Photosynthetic eukaryotes have chloroplasts
    • some from common ancestor
    • others….?


  • Endosymbiosis: Organisms of one species living inside an organism of another species
    • proposed in 1905 but way too crazy

Endosymbiosis Theory: Origin of eukaryotic cells

Does data support Endosymbiosis Theory? YES!!!



  • Mitochondria and chloroplasts are about the size of an average bacterium


  • Both organelles replicate independently by fission, as do bacteria
    • have their own ribosomes to make their own proteins


  • Both organelles have double membranes consistent with the engulfing mechanism


  • Genes of each organelle closely resemble ancestors!
    • cyanobacteria for chloroplasts
    • proteobacteria for mitrochondria

Red and green algae underwent secondary endosymbiosis




  • Algae ingested by other heterotrophic eukaryotes!
    • led to Euglenoids, Diatoms and Brown algae


  • Chloroplasts transferred to other protists
    • results in chloroplasts not from common ancestor!


  • How do we know this happened?
    • HINT: # of membranes around organelles = ____

Euglena: chloroplast with triple membrane

Eukaryotes have diverse sex lives

  • Haplonic life cycles: most life spent with one set of chromosomes (1N)
    • zygote only diploid → meiosis into daughter cells
    • all cells are haploid (some multi-celluar algae, and fungi)


  • Diplonic life cycles: most life with pairs of chromosomes (2N)
    • sex cells (gametes) are haploid via meiosis → then fuse
    • body cells (e.g., animals) are all diploid


  • Asexual life cycles: binary fission
    • body pinches into 2 parts → then mitosis
    • many-single celled protists


  • There are more….
    • parasites require hosts

Life cycles in Eukaryotes: Alternation of Generations


  • Most complex life cycles include an alternation of generations
    • some algae and plants
    • both generations must be multi-cellular


  • AOG = the alternation of multi-cellular haploid and diploid generations
    • diploid (2N) = sporophyte → makes spores
    • haploid (1N) = gametophyte → male/female → make gametes
    • fertilization of gametes makes new sporophyte


  • In some species the different generations look similar while in others they look different

Alternation of Generations: Basic Cycle

Protists matters: Ecology